Moved this from the "Under the hood" forum.


Firstly, this is my first post, so "Hi" to all.

I have a 1973 BMW 2002 running a supercharged S2K engine. It's a Rotrex SC from TTS.

We mapped it for the first time yesterday at Emerald in Norfolk, UK. We managed to achieve 318 bhp and 210 lbft, but although it's still going to be awesome, I thought it would have produced more. I just wondered if those of you out there with SC S2Ks could let me know what power you've managed to achieve. At the moment, we think the issue is likely to be the exhaust.

Any help or info would be really appreciated! Thanks!


Elliott

www.facebook.com/carcrazedfool

 

The 318 Bhp and 210 lbft was achieved with 12psi running the C38-81 Rotrex blower.

//Elliott

 

I can't see FB from work. Can you give more details on your setup like: F20 or F22, exhaust setup, built engine or stock compression etc etc.

 

318 is low for 12psi if you're running stock compression.

I'm making 380whp/240tq at 10/10.5psi. Different kit but you should still be seeing bigger numbers than that

 

I was at 405whp 240tq with my novi1000 comptech kit 15psi f20c

 

Maybe check the compression and plug gap. What size exhaust is on the car?

 

wait wait wait
Dyno numbers depend on what kind of dyno and your elevation, relative humidity, the operator of the dyno,etc. You can see 50 to 100HP difference. So do not be making this guy feel bad.

And exhaust has very little effect on superchargers. Turbo's yes, supercharges no. With a SC if you stuff it in there it will come out. Know many who run over 400WHP and use STOCK exhaust.

I believe a Rotrex is positive displacement type SC not a centrifugal. It is independent of RPM, yes, you get full boost as soon as it turns. So blow off my be handled differently, the curve is also very different. We have apples and oranges here.

 

^^ rotrex is NOT a positive displacement s/c. Just look at his dyno, obviously it's a centrifugal blower since his boost plot rises with rpms.

OP, Your peak torque should be at max rpms. The flattening of torque up top should not be happening. You're gaining boost but maintaining the same amount of torque output? That isn't correct. You're not losing boost, so it's either the tune or something else that's causing the engine to stop making power. Make sure your plugs are gapped correctly for high boost set up, and as mentioned earlier, with SC setups, if you free up the exhaust you may not gain power because you will be losing boost since it is freeing up air flow, the engine can't maintain pressure as well (lamens terms here). So if you have a 50mm dual on the car and it's making 12psi, if you put a 3" single on it, you'll probably make the same power, just at say 10psi. Allowing you to bump boost back up to 12psi and (theoretically) make more power at your previous boost level.
Hope that makes sense, and good luck on finding your missing power!

 

So, off the top of my head the spec is:

Rotrex C38-81
Deutsche Werks 1000cc injectors
TTS mounting kit with 8 row poly vee pulleys
Rotrex S/C oil system, reservoir, filter, cooler rad,
CP Carrillo pistons 10:1
Brian Crowler steel con rods,
Emerald ECU


The rolling road was a Sun Ram 12 with an air temp of 16 deg C at around sea level.

Hope that helps!

 

There is no issue jumping out at me here, from numbers/psi to the typical centrifugal trq curve on the dyno. We all know dynos across the pond show about 100hp less then what us yanks are accustomed to. Also the trq curve typically starts leveling off at about 7800-8200rpm on a centrifugal, wearas the hp curve does not until the last few hundred rpm. Its very much the same as a stock N/A curve, except the boost increase at the top allows a more gradual taper. The dyno sheet layout is more concentrated so the lines look a bit more severe then what you would see on a broader layout as well. If your charger is reaching its efficiency range earlier in the rpms, the trq curve will drop sooner, even though you’re still building boost. On a blower set up that is spinning right in the sweet spot, you may not see any drop in hp and only a little in trq before redline. IAT has an effect, and so does tune, yes. But overall, there isnt anything really abnormal I’m seeing here.

Consider too that your running a lower then stock comp motor, which means it will yield slightly less boost and power per the same pulley size. Its possible you could be more aggressive with your timing advance above 7500rpm, but we don’t know for sure since we don’t know what your timing is. 12psi isnt a lot, so with lowered compression, id expect a pretty aggressive timing allowance. 12psi on a stock comp motor is well within parameters for a safe tune. Nothing wrong with being conservative though, just don’t expect monster power when comparing similar set ups.